1. Field of the Invention
This invention relates to an additive for papermaking which is particularly useful for papermaking in a neutral to alkaline pH region. It enhances the strength of paper and increases the retention of a filler in papermaking using the filler, and the retention of a sizing agent in papermaking using the sizing agent.
2. Description of the Prior Art
The acid sheet forming process using alum has mainly been used heretofore in papermaking. However, according to the acid sheet forming process, the resulting paper is susceptible to degradation and the paper machine undergo corrosion. Moreover, as the papermaking process is carried out in a closed system, the treatment of the white water and waste water, the corrosion of paper machines, etc. become serious problems. For these reasons, sheet formation in a neutral to alkaline pH region has attracted attention as a process superseding the acid sheet forming process.
With this move to higher pH papermaking, it has become possible to use alkaline fillers, such as calcium carbonate, which are available at low costs but cannot be used in the acid sheet forming process. This has led to the requirement for a papermaking additive which can be used in sheet formation in a neutral to alkaline pH region and which can increase paper strength and the retention of a filler and a sizing agent.
Acrylamide polymers have previously been known to have an excellent paper strength increasing effect. It is known that the acrylamide polymers can be rendered of ionic character by, for example, the Hofmann rearrangement reaction, the Mannich reaction, or the copolymerization with a cationic monomer of an anionic monomer. The Hofmann rearrangement reaction product of polyacrylamide has been variously studied because of its excellent properties. But its stability has not been improved and it cannot be free from degradation in properties with time. The Mannich reaction product decreases in cationic character under neutral to alkaline papermaking conditions, and cannot fully exhibit its effect.
Japanese Laid-Open Patent Publication No. 94697/1985 discloses a paper strength increasing agent comprising a water-soluble copolymer derived essentially from (A) a water-soluble cationic monomer, (B) an alpha, beta-unsaturated dicarboxylic acid or its salt, and (C) (meth)acrylamide. When the cationic monomer (A) is an ester typified, for example, by dimethylaminoethyl methacrylate, the cationic property of this copolymer decreases in a neutral to alkaline pH region, and moreover, the copolymer is susceptible to hydrolysis. On the other hand, if the cationic monomer (A) is dimethylaminopropyl (meth)acrylamide or its quaternized product but the alpha, beta-unsaturated dicarboxylic acid (b) is maleic acid, fumaric acid or its salt, the resulting paper strength increasing agent does not fully show its properties when used in the alkaline sheet forming process using calcium carbonate as a filler.